In recent years, various kinds of digital processing for images have been performed in various electronic devices such as a portable telephone, a personal digital assistant (PDA) and the like which have an image display device mounted thereon.
An image display device which is capable of displaying both a vertically oriented image and a horizontally oriented image can change display methods depending on displayed information to improve the visibility of the displayed information. This is an effective means especially for an image display device which is used by easily rotating an image display device body, such as a portable telephone, a PDA and the like.
Such an image display device necessitates image processing in which images are changed between the vertically oriented image and the horizontally oriented image. Various techniques for rotating images in image display devices have been proposed (see, for example, Japanese Patent Application Laid-Open No. 11-69135 (1999), Japanese Patent Application Laid-Open No. 10-307576 (1998), and the like). A variety of prior art techniques related to image display devices have also been proposed (see, for example, Japanese Patent Application Laid-Open No. 2004-56287, Japanese Patent Application Laid-Open No. 2003-114649, Japanese Patent Application Laid-Open No. 2002-82349, and the like).
However, if an attempt is made to rotate an image in the case of the display of a moving image using, for example, NTSC (National Television Standards Committee) video signals, a difference between a scanning direction during the writing of data into an image memory and a scanning direction during the reading of data from the image memory gives rise to a situation such that, in the course of the reading of image data from the image memory, the writing of image data for the next frame into the image memory is started. Thus, the image for the next frame is contained in each frame image constituting the display of the moving image. In other words, an oblique line indicating a boundary between two image areas different in subject conditions appears on a screen.
The appearance of such an oblique line becomes obvious especially when a subject changes significantly on a self light emitting display having a relatively high response speed, such as an organic EL display. In other words, the appearance of the oblique line is common to general displays having a relatively high response speed.